The present invention relates to an aging treatment for stabilizing an operation of a SnO.sub.2 semiconductor gas sensor.
The SnO.sub.2 semiconductor gas sensor detects the gas concentration through the use of the variation of the electric conductivity due to the chemical adsorption of a reducing gas. The SnO.sub.2 semiconductor gas sensor includes a sensor element of the n-type semiconductor sintered material of which the major component is SnO.sub.2.
The above-mentioned SnO.sub.2 semiconductor, gas sensor is widely used in domestic gas leak alarm devices.
Recently, the present assignee has developed microwave oven model numbers R-5000W and R-5800 which include the above-mentioned SnO.sub.2 semiconductor gas sensor, typically a Figaro Gas Sensor TGS#812 or TGS#813 manufactured by Figaro Engineering Inc., for controlling a cooking operation. In this microwave oven, the oven cooking and the grill cooking are conducted by a sheath heater disposed in an oven cavity, and the microwave cooking is conducted through the use of a microwave energy of 2,450 MHz derived from a magnetron. The cooking control is automatically conducted through the use of an output signal derived from the gas sensor once the operator actuates a desired cooking command switch.
A typical control circuit of the above-mentioned microwave oven is described in copending U.S. patent application Ser. No. 71,179, COOKING UTENSIL CONTROLLED BY GAS SENSOR OUTPUT, filed on Aug. 31, 1979 now U.S. Pat. No. 4,311,895, by Takeshi Tanabe and assigned to the same assignee as the present application. The British counterpart was published on Aug. 28, 1980 and bears a publication number 2,040,502 The German counterpart is DOS No. 2,935,862.
To perform reliable cooking control, the gas sensor must operate stably. However, the conventional gas sensor shows a remarkable variation in its detection characteristics. Further, the conventional gas sensor manufacturing method is not suited for mass production.
Accordingly, an object of the present invention is to stabilize an operation mode of a SnO.sub.2 semiconductor gas sensor when it is used in a microwave oven.
Another object of the present invention is to provide a novel aging treatment for stabilizing the operation of a SnO.sub.2 semiconductor gas sensor.
Still another object of the present invention is to provide an aging treatment of a SnO.sub.2 semiconductor gas sensor suited for mass production.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a SnO.sub.2 semiconductor gas sensor is treated to age in a gas ambience including a gas component which is generated in an apparatus in which the SnO.sub.2 semiconductor gas sensor is desired to be installed.
In a preferred form, to provide a SnO.sub.2 semiconductor gas sensor to be used in a microwave oven, the SnO.sub.2 semiconductor gas sensor is aged in a dimethyl siloxane gas ambience, the dimethyl siloxane being developed from a heated silicon compound. The aging period is thirty minutes through two hours. In another preferred form, the dimethyl siloxane gas ambience is held at a high temperature, for example, 150.degree. C. to 250.degree. C.